﻿using System;

namespace DataStructures.Probabilistic
{
    public class CountMinSketch<T> where T : notnull
    {
        private readonly int[][] sketch;
        private readonly int numHashes;

        /// <summary>
        /// Initializes a new instance of the <see cref="CountMinSketch{T}"/> class based off dimensions
        /// passed by the user.
        /// </summary>
        /// <param name="width">The width of the sketch.</param>
        /// <param name="numHashes">The number of hashes to use in the sketch.</param>
        public CountMinSketch(int width, int numHashes)
        {
            sketch = new int[numHashes][];
            for (var i = 0; i < numHashes; i++)
            {
                sketch[i] = new int[width];
            }

            this.numHashes = numHashes;
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="CountMinSketch{T}"/> class based off the optimizing error rate
        /// and error probability formula width = e/errorRate numHashes = ln(1.0/errorProp).
        /// </summary>
        /// <param name="errorRate">The amount of acceptable over counting for the sketch.</param>
        /// <param name="errorProb">The probability that an item will be over counted.</param>
        public CountMinSketch(double errorRate, double errorProb)
        {
            var width = (int)Math.Ceiling(Math.E / errorRate);
            numHashes = (int)Math.Ceiling(Math.Log(1.0 / errorProb));
            sketch = new int[numHashes][];
            for (var i = 0; i < numHashes; i++)
            {
                sketch[i] = new int[width];
            }
        }

        /// <summary>
        /// Inserts the provided item into the sketch.
        /// </summary>
        /// <param name="item">Item to insert.</param>
        public void Insert(T item)
        {
            var initialHash = item.GetHashCode();
            for (int i = 0; i < numHashes; i++)
            {
                var slot = GetSlot(i, initialHash);
                sketch[i][slot]++;
            }
        }

        /// <summary>
        /// Queries the count of the given item that have been inserted into the sketch.
        /// </summary>
        /// <param name="item">item to insert into the sketch.</param>
        /// <returns>the number of times the provided item has been inserted into the sketch.</returns>
        public int Query(T item)
        {
            var initialHash = item.GetHashCode();
            var min = int.MaxValue;
            for (int i = 0; i < numHashes; i++)
            {
                var slot = GetSlot(i, initialHash);
                min = Math.Min(sketch[i][slot], min);
            }

            return min;
        }

        private int GetSlot(int i, int initialHash) => Math.Abs((i + 1) * initialHash) % sketch[0].Length;
    }
}
